Traveling-wave tube



Nov. 21, 1961 Filed March 9, 1959 D. J. BATES TRAVELING-WAVE TUBE 6 Sheets-Sheet l D. J. BATES TRAVELING-WAVE TUBE Nov. 21, 1961 6 Sheets-Sheet 2 Filed March 9, 1959 Ana/Wm 04/40 475475;, 6/

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Nov.-21, 1961 D. J. BATES 3, ,04

TRAVELING-WAVE TUBE Filed March 9, 1959 6 Sheets-Sheet 3 Nov. 21, 1961 D. J. BATES 3, 4

TRAVELING-WAVE TUBE Filed March 9, 1959 6 Sheets-Sheet 4 lira-.6.

Nov. 21, 1961 D. J. BATES TRAVELING-WAVE TUBE 6 Sheets-Sheet 5 Filed March 9, 1959 //V/iA 70 04/40 E5475), ayk) P Nov. 21, 1961 Filed March 9, 1959 D. J. BATES TRAVELING-WAVE TUBE 6 Sheets-Sheet 6 3,910,047 TRAVELING-WAVE TUBE David J. Bates, Rolling Hills, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Mar. 9, 195% Ser. No. 798,292 8 Claims. (Cl. SIS-3.5)

This invention relates to electron devices, particularly to periodically loaded waveguides utilized as slow-wave structures.

It is frequently desirable to provide electromagnetic interaction between a stream of charged particles and an electromagnetic wave for purposes of exchanging energy from one to the other. For example, in a traveling-wave tube a stream of electrons is utilized to amplify an electromagnetic wave which is propagated in a medium near the electron stream. In a linear accelerator, as another example, an electromagnetic wave is utilized to accelerate a stream of charged particles.

Usually, in such devices the electromagnetic wave is caused to propagate with a component of velocity along the path of the stream of charged particles which is approximately equal to that of the stream of charged particles. It is therefore seen that the component of velocity of the electromagnetic wave must be considerably less than the speed of light.

A particularly useful structure for providing the required reduced component of velocity and the desired interaction between the stream of particlesand the slowwave is a microwave waveguide structure which is periodically loaded with conductive irises to form a series of microwave cavities along the length of the waveguide. A central axial opening is then provided through the irises to provide a passageway for the stream of charged particles which may then interact with microwave energy in the series of cells. In the so-called folded Waveguide or coupled cavity space harmonic circuit, the cells are intercoupled, usually by a single off-center coupling hole. In the past, it has been the usual practice to alternate the angular or azimuthal position of successive ones of these off-center coupling holes so that microwave energy may excite each cell and be propagated on to the next without, however, having a straight-through path of propagation which would be possible if the coupling holes were not alternately offset. It is the usual practice to offset each hole 180 with respect to those adjacent to it. This scheme has been utilized primarily because it has been known that the width of the passband of the fundamental mode of propagation is maximum for this orientation, the field configuration of this mode is not adversely affected by the alternation, and because of the resultant antisymmetry which, it has been considered, might reduce the excitation and propagation of some spurious higher order modes, where higher order designates modes of interaction higher than that of the desired fundamental passband.

I have discovered, however, that when attempting to develop a practical, reproducible, high gain, broadband, high powered traveling-wave tube amplifier device this conventional autisymmetrical distribution of coupling holes gives rise to undesirable higher-order modes of interaction between the electron stream and the traveling circuit waves.

In particular, it has now been found that when the coupling holes are arranged in a symmetrical, antisymmetrical or axially registering scheme of distribution along the length of the traveling-wave tube at any instant of time, azimuthally varying and other higher-order modes of excitation can exist in the individual cavities. An azimuthally varying mode provides, in'traversing within States atent a cavity along a circle at a given instant of time, a variation of the electric field as a function of angular position within the cavity. It has been found further that at least one of these modes may interact with the electron stream and cause deleterious oscillations and other effects.

It is therefore an object of the present invention to provide a slow-wave structure which is not subject to the disadvantages of the prior art discussed above.

It is another object to provide a periodically loaded waveguide type of slow-wave structure which minimizes interaction between undesirable azimuthally varying and other modes of excitation and the associated stream of charged particles.

It is another object to provide a traveling-wave tube having an interconnected cavity type of slow-wave structure in which the azimuthally varying modes of excitation are distributed for greatest efliciency.

Another object of this invention is todisturb the passband characteristics of a slow-wave structure sothat the coupling between adjacent cavities and the axial interaction field configurations are altered to provide improved operation.

A further object of the invention is to disturb thephase shift characteristics of a periodic structure so that oscillations in the operating voltage range are minimized.

Briefly, these and other objects are achieved in accordance with the present invention by providing in an interconnected cavity type of slow-wave structure a dis tribution of the off-center coupling holes along the length of the slow-wave structure, which distribution is angularly asymmetric, that is, it is neither symmetric, antisymtribution of the coupling holes may be the most favorable. The novel features of this invention, as well as the invention itself, both as to its organization and method of operation, will best be understood from the following description, taken in conjunction with the accompanying drawings, in which like reference numerals refer to 7 like parts, and in which: I

FIG. lis anvoverall view of a traveling-wave tube constructed inaccordance with the present invention, the.

view being partly in longitudinal section and partly broken y;

FIG. 2 is a detailed longitudinal sectional view of a portion of the tube illustrated in FIG. 1;

FIG. 3 is an axially exploded view of a set of typical elements included in the structure of an embodiment of the present invention;

FIG. 4 is a typical cross-sectional view of structures heretofore employed, useful in explaining certain features of the invention;

FIG. 5 is a schematic plot of the radio frequency electric fields in a 2mr mode of excitation of the structure of FIG. 4; i

.FIG. 6 is a schematic plot of the radio frequency electricfields in a (-2n+1)1r mode FIG. 7 is a view of three successive elements of a slow-wave structure arranged in accordance with the present invention; and

If a uniform magnetic field is being utilized, a zig-zag dis- FIGS. 8 and 9 are axially exploded views illustrating particular arrangements of successive elements of slowwave structures constructed in accordance with the present invention.

Referring to the drawings and their description, a number of features are shown for purposes of completeness of description of a practical and operational traveling-wave tube according to the present invention, which features are not claimed in the present application. These features are claimed and described more fully in prior filed applications assigned "to the assignee of the present application, for example, Periodically Focused Traveling-Wave Tube, by D. I. Bates, H. R. Johnson and O. T. Purl, Serial No. 764,884, filed October 2, 1958, to which reference may be made for a more detailed understanding.

Referring with more particularity to FIG. 1, there is shown a traveling-wave tube 12 utilizing a plurality of annular disc-shaped focusing magnets 14. In the example of this figure, these are permanent magnets and are diametrically split, as shown in later figures, to permit their being easily slipped between assembled adjacent ones of a series of ferromagnetic pole pieces 16, which are also shown in more detail in the later figures. The system, including pole pieces 16 and magnets 14, forms both a slow-wave structure and envelope 18.

Coupled to the right-hand or input end of the slowwave structure 18 is an input waveguide transducer 20 which includes an impedance step transformer 22. A flange 24 is provided for coupling the assembled traveling-wave tube 12 to an external waveguide or other microwave transmission line (not shown). The construction of the flange 24 includes a microwave window (not shown) transparent to radio frequency energy but capable of maintaining a pressure differential for maintaining a vacuum within the traveling-wave tube 12. At the output end of the tube 12, shown in the drawing as the left-hand end, an output transducer 26 is provided which is substantially similar to the input impedance transducer 20. An electron gun 28 is disposed at the right-hand end, as shown in the drawing, of the traveling-wave tube 12 and comprises a cathode 30 which is heated by a filament 32. The cathode 30 has a small central opening 34 to aid in the axial alignment of the gun assembly with the remainder of thetraveling-wave tube 12. The cathode 30 is secured about its periphery by a cylindrical shielding member 36 which is constructed in a manner to fold cylindrically, symmetrically back upon itself to form a double cylindrical shield and an extended thermal path from the cathode 30 to its outer supporting means. Such support and an electrical, highly conductive path to the cathode is thus achieved while providing considerable thermal insulation for the cathode and filament due to the extended or tortuous path for heat conduction, as well as because of the multiple cylindrical shielding against radiant heat which is provided by the cylinders shown. For additional details of this type of gun construction see the patent to J. A. Dallons, No. 2,817,039, entitled Cathode Support, issued December 17, 1957.

A focusing electrode 38, supports thecylindrical shielding member 36. The focusing electrode 38 is generally maintained at the same potential as that of the cathode 30 and is shaped to focus the electron stream emitted by the cathode in a well-collimated, high perveance beam of electrons which traverses the slow-wave structure 18 and electromagnetically interacts with microwave energy being propagated therealong. Reference may be made to Patents No. 2,811,667 and No. 2,817,033 for a more detailed explanation of such structures. The focusing electrode 38 is in turn supported by a hollow cylindrical support 40 which extends from the periphery of the focusing electrode to the right-hand end of the traveling-wave tube 12. Its opening is hermetically sealed with a metal-to-ceramic seal 42 by means of a sealing flange 44 made of a material having a low ooeflicient of thermal expansion, such as Kovar. The right-hand extremity of the cylindrical support 40 is supported by an annular flange member 46, which also may be of Kovar, and which is sealed in turn to a hollow ceramic supporting tube 48. The ceramic tube 48 further thermally insulates the inner intensively heated members of the electron gun 28 and also provides electrical insulation between the cathode-beam focusing assembly and the higher potential accelerating anode 52. Substantially encasing the electron gun 28 and secured to the central or radio frequency structure of the travelingwave tube 12 is a hollow cylinder 50, which may be Kovar, to which is sealed the ceramic cylinder 48, thus completing the vacuum envelope about the right-hand end of the traveling-wave tube 12.

At the left-hand end of the tube 12, as viewed in FIG. 1, there is shown a cooled collector electrode 60 which has a conically-shaped inner surface 62 for collecting the electrons from the high power electron stream and dissipating their kinetic energy over a large surface. The collector electrode is supported within the end of a water jacket cylinder 64 which is in turn supported by an end plate 66.

A water chamber 68 is thus formed between the outer surface of the collector electrode 62 and the inner cylindrical surface of water jacket 64. A water input tube '70 supplies cool water to this chamber and a water output tube 72 exhausts the heated water out of the water chamber 68. Thus, considerable power may be dissipated without destruction of the collector electrode. Although water has been specified, obviously, other liquids or gases may be used as coolants.

Theend plate 66 is sealed to a supporting cylinder 74, which may be of Kovar, and which is in turn sealed to a ceramic insulating cylinder 76. This ceramic insulating cylinder 76 is sealed at its opposite end to another Kovar supporting cylinder 78, which is in turn supported and sealed to the slow-wave structure end disc 86. The collector 62, the end plate 66, the supporting cylinders 74 and 78 and the ceramic insulating cylinder 76 are all coaxially supported in alignment with the axis of the traveling-wave tube 12.

For vacuum pumping or out-gassing the traveling-wave tube 12, a double-ended pumping tube 86 is connected to both of the input and output'waveguide transducers 20 and 26. Out-gassing during bake-out of the entire traveling-wave tube 12 is thus achieved as rapidly as possible. After the out-gassing procedure, the tube 86 is separated from the vacuum pumping system by pinching off the tube at the tip 88.

The traveling-wave tube of the present invention may be severed into a number of amplifying sections 90, 92, 94, 96 and 98. Each of the amplifying segments or sections is isolated from the others by an isolator or termination section 100, 102, 104 or 106. The structure of these isolating sections will be discussed in more detail in connection with FIG. 2. It suffices at this point to describe their function generally as providing a substantially c0mplete radio frequency isolation between adjacent sections of the slow-wave structure 18 while at the same time allowing the electron stream to pass straight through the entire length of the traveling-wave tube 12. Each amplifying section thus provides an optimum gain while providing freedom from oscillations due to regeneration. The loss in gain due to each of these isolation sections is of the order of a few decibels and is a low price to pay for the large overall gain and power-handling capabilities of a traveling-wave tube constructed in accordance with the present invention. It should be noted that although the isolation sections provide substantially complete radio frequency isolation between adjacent amplifying sections, the electron stream is modulated at the output of each amplifying section. The stream thus modulated, as it enters the subsequent amplifying section, launches a new wave therein which is further amplified by the interaction between the new traveling wave and the electron stream. Thus, there is provided unidirectional coupling through the electron stream between adjacent amplifying sections.

Referring with more particularity to FIG. 2, there is shown a detailed sectional view of a portion of the traveling-wave tube of FIG. 1. The ferromagnetic pole pieces 16 are shown to extend radially inwardly to approximately the perimeter of the axial electron stream. Disposed contiguously about the electron stream in each case is a short drift tube 110. The drift tube 110 is in the form of a cylindrical extension or lip protruding axially along the stream from the surface of the pole piece 16.

Adjacent ones of the drift tubes 119 are separated by a gap 112 which functions as a magnetic gap to provide a focusing lens for the electron stream and also an electro-magnetic interaction gap to provide interaction between the electron stream and microwave energy traversing the slow-wave structure.

At a radial distance outwardly from the drift tubes 119 each of the pole pieces 16 has a second short cylindrical extension 114 protruding from its surface. The extension 114 provides an annular shoulder concentric about the axis of the tube for aligning the assembly of the component elements of the slow-wave structure 18. Disposed radially within the extension 114 is a conductive, nonmagnetic circuit spacer 116 which has the form of an annulus or ring having an outer diameter substantially equal to the inner diameter of the cylindrical extension 114. The axial length of the spacer 116 determines the axial length of the micro-wave cavities 118 which are interconnected along the length of the slowwave structure 18. It is thus seen that the slow-wave structure may be assembled and self-aligned by stacking alternately the pole pieces 16 and the spacers 116. Each spacer 116 has two annular channels 120 in which, during the stacking procedure, a sealing material, such as a brazing alloy, is placed. When the slow-wave structure 13 is assembled, it may be placed in an oven within a protective nonoxidizing atmosphere and heated so that the brazing alloy in the channels 121 melts and fuses or brazes the adjacent members of the slow-wave structure 18 together to form a vacuum tight envelope. The spacers 116 are fabricated of a nonmagnetic material, such as copper, thus providing a highly conductive cavity wall while not magnetically shorting out the focusing gaps 112. The entire interior surfaces of the cavities are preferably plated with a highly conductive material, such as a thin silver or gold plating 121.

For interconnecting adjacent interaction cells, a coupling hole 122 is provided in each of the ferromagnetic pole pieces 16, the more detailed shape and orientation of which will be described in connection with the description of subsequent figures below. Also .disposed between adjacent pole pieces 16 are the focusing magnets 14 which are annular in shape and fit angularly or azimuthally symmetrically about the cylindrical shoulder extensions 114. The magnets 14 may be diametrically split to facilitate their being applied to the slow-wave structure 18 after it has been otherwise assembled. The axial length of the magnets 14 is substantially equal to the axial spacing between adjacent pole pieces 16, and their radial extent is approximately equal to or may be, as shown, greater than that of the pole pieces 16. To provide the focusing lenses in the gaps 112, adjacent ones of the magnets 14 are stacked with opposite polarity, thus causing a reversal of the magnetic field at each successive lens along the tube.

Referring to a typical isolator section 100, there is shown a substantial continuity of the pole piece-magnetspacer assembly. However, the pole pieces 124 at either end of the isolator section and the spacer 126 are somewhat modified, with respect to pole piece 16 and spacer 116 respectively. The attenuating material, which may be in the form of lossy ceramic buttons 128, extends from within a coupling hole 122 through the special spacer 126 and partially into the wall of the pole piece 124 opposite the coupling hole. The spacer 126 forms a pair of modified cavities 130 which lie opposite respective ones of the coupling holes 122. and which are substantially filled with the lossy attenuating material.

The two cavities 136 are'substantially isolated from each other by a short circuiting vane 132 and are isolated from interaction with the electron stream by means of a central portion of the special spacer which has the form of a ring having substantially the same radial dimensions as the drift tubes and which extends between two of the drift tubes 110 as shown, in a manner to substantially shield the electron stream from the slow-wave structure in the region of the isolator section 100.

Along the length of the slow-wave structure 18, individual ones of the pole pieces 16 are spaced by axial distances as represented by a, b, c and d. In a preferred arrangement of the traveling-wave tube of the present invention, these distances and the associated length of the spacers 116 may be slightly varied with respect to each other so that the effective axial length of the interaction cavities is successively increased toward the output or collector end. This is done in order to decrease the axial phase velocity of the traveling waves so that the desired interaction between the electron stream and the traveling waves will continue to a maximum degree even though the electrons are slowed'down toward the collector end.

Referring to FIG. 3, one set of the plurality of pole pieces, magnets and spacers is shown for purposes of describing more clearly how the individual elements of,

the slow-wave structure 18 are fabricated and assembled. A typical pole piece 16 is shown twice in the figure, once in plan and once in side elevation. A typical magnet 14 and a typical spacer 116 are shown in side elevation only.

Referring to the side elevation view of the pole piece 16, the orientation of the pole piece 16 concentrically about the electron stream is shown. Substantially immediately surrounding. the electron stream is the short drift tube 110 which extends axially in both directions normal to the plane of the pole piece 16. The remainder of the pole piece extends radially outwardly from the drift tube 110 as shown. Positioned radially in between these two extremes are the cylindrical shoulder extensions 114 which extend axially outwardly from both faces of the pole piece 16.

The outer diameter of the cylindrical extension 114 supports the focusing magnet 14 coaxially about the electron stream, while the inner diameter of the extension 114 rests against the outer periphery of the spacer 116. The inner diameter of the spacer 116 determines the outer dimension of the interaction cell which is formed between adjacent ones of the pole pieces 16. Before assembly, a sealing material is placed in the channels 120, which are continuous annular grooves in the end surfaces of the spacers 116.

An oif-center coupling hole 122 is provided through each of the pole pieces 16 to provide the transfer of radio frequency energy from cell to cell along the slow-wave structure 18.

The size, shape and orientation of the coupling hole 122 may be more clearly seen inthe plan view thereof at the left-hand end of FIG. 3. The drift tube 119 is shown as having an inner radius r slightly larger than the radius of the electron stream and having an outer radius r; which substantially defines the inner radius of the interaction cell. The kidney-shaped coupling hole 122 may be formed by an end mill having a diameter extending from r to 1' The end mill is pressed through the thickness of the pole piece 16 centered upon the arc of a circle 132. The endrnill, or preferably the work, may be swung along this arc keeping its center on the circle 132. The work is rotated through an arc of angle a where a may be any angle between zero degrees and, for example, approximately 60. Thus, the kidney-shaped coupling hole 122 lies between a radius r and 21; diameter r r Disposed radially outwardly from the coupling hole 122 is a cylindrical shoulder extension 114, the inner radius of which is designated r and is substantially equal to the outer radius of the spacer 116. The inner radius r,-, of the spacer 116 determines the outer dimension of the radio frequency interaction cell. The outer radius of the extension 114, designated as r is substantially equal to the inner radius of the magnet 14. The outer radius of the pole piece 16 is designated r and the outer radius of the magnet 14 is designated r For angular alignment purposes during assembly, one or more sets of holes 134 are provided through the pole pieces 15 to hold them in a predetermined angular position with respect to each other. be provided on the periphery of each of the pole pieces 16 in order that one may always know from an observa: tion of the outer surface of the assembled tube what the angular orientation of each pole piece is. In the example described here, the notch is provided opposite the center of the kidney-shaped coupling hole 122.

In the operation of the traveling-wave tube 12, microwave energy traverses from right to left along the slowwave structure, being amplified first in section 98 due to its interaction with the electron stream. Near the output of this amplifying section, the traveling wave has grown and has caused considerable density modulation in the electron stream. At the first isolator section, section 106 in the drawing, the radio frequency energy in the slowwave structure 18 is substantially completely absorbed. However, the modulated electron stream passes on into the next amplifier section, section 96, where it launches a and has circular ends of new traveling Wave in that section. The new traveling wave grows and is amplified by the electron stream until reaching its output end at the isolator section 104. The electron stream is further modulated and the RF energy in the slow-wave structure is again completely absorbed. This procedure is repeated until the highly modulated electron stream enters the output amplifier section 90 through the isolator section 100 and launches a high energy traveling wave upon the output section 90 of the slow-wave structure 18. The output of this final section is fed into the output waveguide through the transducer 26.

The isolator sections 100, 102, 164 and 1% each represent a loss of a few decibels of amplification. However, overall they vastly increase the amount of power amplification or gain which may be achieved in a single traveling-wave tube. The isolation sections solate adjacent amplifying sections, thereby to preclude instability and oscillations due to reflections and to too great an amplification in a single traveling-wave tube section.

The following affords a description of the radio frequency fields of the first, higher order, azimuthally varying mode.

Referring to FIGS. 4, and 6, an illustration is presented of the angular variation of the radio frequency electric fields along the length of a periodically loaded or interconnected cell-type of slow-wave structure of the type heretofore used. FIG. 4 is a typical cross-section of a structure shown in longitudinal section in FIGS. -5 and 6. As pointed out previously, it has generally been the practice to alternately stagger the coupling holes 150 by 180. This angle has been chosen because such a staggered relationship of the coupling holes produces the largest bandwidth of the fundamental mode but does preclude a straight-through or optical path mode of propagation. Further, a tube with this type of angular antisymmetry was deemed to afford very simple intuitive analysis of its electromagnetic operation. Also, such a tube is relatively easily constructed because of having only two angular orientations for the coupling holes. These figures also illustrate an axial opening 152 in each of the loading discs 154 for passage of the axial electron stream.

The illustrations of FIGS. 4, 5 and 6 are schematic and A reference notch 136 may I are intended to be general. They refer, for example, equally to the periodically focused embodiment of FIG. 1, to a periodically focused embodiment in which the pole pieces do not form a part of the radio frequency interaction circuit and to an interconnected cell-type of traveling-wave tube which is focused with a uniform magnetic field as by a large external solenoid or permanent magnet.

The axis of the tubes shown is identified as the z axis in the figures, the vertical direction is represented as the y axis in the figures and the horizontal direction orthogonal to the z axis is represented in the figures as the x axis.

Therefore, in FIG. 4 an end-on view of the yz and x-z planes is shown while the plane of the figure is the xy plane. The plane of the drawing in FIGS. 5 and 6 is the y-z' plane.

3 Referring now particularly to FIG. 5, there is shown by means of the vectors 156 the radio frequency electric field distribution in the vz plane when the phase shift per interaction cell 158 is equal to 21m radians where n is any integer. The electric fields are in phase and in the same direction throughout the length of the structure. However, the electric fields throughout the axial plane perpendicular to the drawing, that is, throughout the x-z plane, are everywhere zero. Thus, if a test charge is moved around a cell at constant z, at constant time and at a constant radius within the cell, it would go through a periodical angularly sinusoidal varying of electric field. It is this azimuthally varying electric field and its interaction with the electron beam which may cause deleterious oscillations in a high gain traveling-wave tube, the elimination of which oscillations is among the objects of the present invention.

Referring in particular to FIG. 6, there is shown another mode of excitation in such a periodically loaded slow-wave structure. The array of the vectors 161 illustrates the radio frequency electric field distribution when the phase shift per interaction cell is equal to (2n+1)1r radians. .It can beseen that the distribution in each cell is the mirror image of that in the adjacent cells. Within each cell, however, there still exists the same type of angular antisymmetry as discussed above in connection with FIG. 5.

There are an infinity of other phase shifts between the (2i1|1)1r and the Zmr cases, the illustration of which would be difficult or impracticable to achieve, but they too will exhibit an azimuthal antisymmetiy and may interact deleteriously with the electron stream.

Referring to FIG. 7, a generalized arrangement of three typical pole piece elements 16 having a nonantisymmetrical distribution of coupling holes 122 is shown. The pole pieces 16 shown are of the character as utilized in a traveling-wave tube constructed in accordance with the embodiments of FIGS. 1 through 3. The three pole pieces shown are adjacent pole pieces but are depicted-in an axially exploded arrangement for clarity. Again, the tube axis is designated as the z axis and the vertical direction is designated as the y axis with the x axis coming out of the plane of the paper and being orthogonal to both y and z.

The coupling holes in accordance with the present invention are rotated with respect to adjacent ones by an angle other than The purpose, as discussed above, of such rotation is to destroy the otherwise existent symmetry or antisymmetry of the azimuthally varying modes of excitation discussed in connection with FIGS. 5 and 6. The angle b is a designation of the angular displacement of the center of the kidney-shaped coupling hole 122 from the y z plane. The angle c designates, similarly, the displacement of the kidney-shaped hole 122 of the third pole piece from the y-.z plane. A fourth, et seq., pole piece would be similarly rotated by other angles. Actual practical examples of the magnitudes of the angles b and 0 will be discussed in connection with subsequent figures. It is sufficient here to note that the angles of rotation should be selected from a consideration of providing an 9 asymmetry such as to preclude a synchronous interaction between the traveling waves and the electron stream; further, a distribution should be chosen which does not inhibit excitation and propagation of the desired fundamental mode.

FIGS. 8 and 9 are generalized schematic views of exploded arrangements of coupling irises 162 of a periodically loaded slow-wave structure, the remaining details of which are not shown. The type of loading disc or iris 152 shown, each with a coupling hole 163, is intended as a generic illustration of such elements, Whether, for example, they be part of a periodically focused tube or of a uniformly focused tube. It may be seen from FIG. 8 that a zig-zag distribution of coupling hole rotation is utilized. This is found to be particularly useful when such a struc ture is focused with a uniform magnetic field. The zigzag constitutes an angle of zero with respect to the yz plane for a first iris 162, an angle of +120 for the second disc, an angle of zero for the third, an angle of -120 for the fourth and an angle of zero for the fifth, et cetera.

FIG. 9 illustrates a distribution of the rotational positions of the coupling holes 163 which is particularly useful for periodically focused traveling-wave tubes. In this embodiment, a continuous spiraling distribution is shown, that is, the angle of the coupling hole 163 of the first iris 162 is zero degrees with respect to the y-z plane, the second is rotated 120 with respect thereto, the third is rotated an additional 120, the fourth is rotated still another l20, the fifth is rotated still another 120, et cetera.

Such distributions for rotating schemes are merely examples of many which have proven to be beneficial in reducing interaction between the electron beam and an azimuthally varying or other undesired electric field distribution in the coupled cavity type of slow-wave circuit. This reduction of interaction eliminates many deleterious oscillations and thereby produces a much more stable tube. Increased stability in turn permits traveling-wave tubes of this character to be operated at substantially higher power levels and at substantially greater gain such as to make practical their utilization in many applications where, but for the asymmetry hereinabove discussed, such tubes would be impractical.

It may be noted that the method for reducing the interaction is actually to change the passband characteristics of the azimuthally varying and other higher order modes and also to cause the electron beam and the radio frequency electric fields in the interaction gap to be asynchronous in a given voltage operating range. In other Words, the impedance of the particular space harmonics, at which frequency oscillations generally occur, is reduced to such a low value that oscillation will not occur in a practical voltage operating range.

A number of ways have been shown for destroying the particular type of symmetry characteristics of these modes. A random variation of the coupling hole rotation is desirable in some cases; however, systematic variations like the examples of FIGS. 8 and 9 have been found to be the most practical as regards assembly of the tubes and analysis of the operation. There are alternate methods of construction which do not necessarily involve ro tation of the coupling holes. For example, the asymmetry is introduced into the periodic circuit by the addition of probes and loops, or by machining asymmetries into the cavities, the objective in any case being to identify and analyze the higher order modes which cause deleterious oscillation in a given tube and in a particular manner to perturb the interaction fields and passbands thereof by introducing a predetermined azimuthal asymmetry. Also, this technique and construction is applicable to slow-wave circuits other than those described above. Other space harmonic circuits, for example, interdigital, so-called Hines and modified Hines structures, as well as 10 some fundamental structures, have a symmetry or antisymmetry that can cause oscillations due to the excitation of the azimuthally varying and other higher order modes. Such structures may also employ arrangements in accordance with this invention.

What is claimed is:

1. A traveling-wave device comprising: a periodically loaded waveguide structure having spaced conductive walls defining interaction cells along the axis of the device, each of said conductive walls having an aperture therethrough for coupling electromagnetic energy between adjacent ones of said interaction cells, each of said apertures having a predetermined azimuth angle about said axiswith respect to an axial reference plane, successive ones of said apertures having a different such azimuth angle, the indicated difference angle being other than mr radians Where n is any integer.

2. A traveling-Wave device comprising: a periodically loaded waveguide structure having a linear axis and a longitudinal reference plane including said axis, said waveguide structure including spaced conductive vanes transverse to said axis and defining a series of interaction cells therealong, each of said vanes having a coupling aperture therethrough, each of said coupling apertures having a definitive azimuthal disposition about said axis with respect to said reference plane, said azimuthal dispositions of successive ones of said coupling apertures being different and by an angle other than mr radians Where n is any integer.

3. A traveling-wave tube having a longitudinal axis comprising: a periodically loaded waveguide type slowwave structure including loading vanes disposed along and transverse to said axis and defining a series of interconnected interaction cells, each of said loading vanes having an aperture therethrough for coupling electromagnetic energy between adjacent ones of said cells, each of said apertures being disposed off-axis and at a predetermined azimuth angle about said axis with respect to an axial reference plane, the apertures associated with adjacent ones of said loading vanes having diiferent azimuth angles with respect to said reference plane, said different azimuth angles having a difference absolute magnitude of substantially more than zero radians and substantially less than 1r radians.

4. An electromagnetic structure for providing interaction between a stream of charged particles projected along a predetermined path and radio frequency electromagnetic energy comprising electrically conductive magnetic means for providing a series of electromagnetically intercoupled interaction cells arranged along said path in electromagnetic interaction relationship with said stream of charged particles and including a series of magnetic lenses along and substantially immediately contiguously about said stream for focusing and constraining it to flow along said path, and angularly with respect to said path, nonsymmetric and nonantisymmetric and nonaxially in line, means associated with said series of cells to substantially preclude interaction between said stream and an azimuthally varying mode of radio frequency excitation of said cells.

5. In a traveling-wave tube comprising means for projecting a stream of electrons along a predetermined path and for reducing interaction between an azimuthally varying mode of excitation of said structure and said stream, means for providing a series of magnetically intercoupled interaction cells arranged along and in electromagnetic interaction relationship with said electron stream and for providing a series of magnetic lenses along and contiguously about said stream for focusing and constraining it to flow along said path, said interaction cells having at each end thereof a paramagnetic disc element extending radially from said electron stream to a predetermined outer diameter and a spacer element hermetically sealed between adjacent ones of said disc elements and being relieved to form the interaction volume about said elec tron stream and being disposed concentrically thereabout, the outer extremity of said spacer element being substantially less distant from said stream than said pre determined outer diameter of said disc; a plurality of permanent magnets each disposed between a different adjacent pair of said disc elements and radially between the associated annular spacer and approximately said predetermined outer diameter, and asymmetric with respect to said path, means associated with said interaction cells to preclude an angular symmetry and antisymmetry of said azimuthally varying mode of excitation.

6. In a periodically focused traveling-wave tube utilizing a periodically loaded waveguide slow-wave structure in which a plurality of conductive transverse loading vanes define a series of interaction cells along the axis of the tube for providing electromagnetic interaction between radio frequency energy propagated therealong and an associated electron stream projected along said axis, said stream due to its being periodically focused including electrons having an oscillatory component of angular motion about said path, said conductive vanes having kidney-shaped coupling holes therethrough for electromagnetically intercoupling adjacent ones of said series of cells, said holes being disposed through said vanes &- axis and at prescribed azimuth angle about said axis with respect to a predetermined axial reference plane, said prescribed angle associated with'a particular one of said vanes with respect to an adjacent preceding one of said conductive vanes being successively rotated about said axis in a constant sense of rotation by an angle substantially more than zero and substantially less than mr radians where n is any odd integer whereby synchronous interaction between the fields of said electron stream and said azimuthally varying mode is substantially preeluded.

7. A high powered periodically focused traveling-wave tube comprising means for projecting a stream of electrons along a predetermined path; means for providing a series of electromagnetically intercoupled interaction cells arranged along and in electromagnetic interaction relationship with said electron stream and also providing a series of magnetic cooperating lenses along'and contiguously about said stream for focusing and constraining it to flow along said path, individual ones of said interaction cells having at each end thereof a ferromagnetic disc element extending radially from said electron stream to a predetermined outer diameter; a plurality of spacer elements each hermetically sealed between adjacent ones of said disc elements and being radially inwardly relieved to form the interaction volume about said electron stream and being disposed substantially concentrically tbereabout, the outer extremity of each spacer element being radially substantially less distaut from said stream than said predetermined outer diameter of said disc; a plurality of permanent magnets for focusing said stream disposed between adjacent ones of said discs elements symmetrically radially outwardly therefrom, adjacent ones of said interaction cells being interconnected by means of a coupling hole relieved through said disc elements radially between said electron stream and the inner diameter of said spacer elements, said coupling holes being angularly oriented in a predetermined pattern to preclude interaction between an azimuthally varying mode of excitation of said series of saidinteraction cells and said electron stream, said predetermined pattern of angular orientation being angularly asymmetric about said path along said tube to minimize synchronous interaction between the field of said electrons and those of said azimuthally varying mode of excitation.

8. In atraveling-wave tube focused at least in part by a substantially uniform axial magnetic focusing field and utilizing a periodically loaded waveguide slow-wave structure defining a series of radio frequency interaction cells disposed along the axis of the tube for providing electromagnetic interaction between radio frequency energy propagated along said cells and an associated axial electron stream which includes electrons having an angular component of motion about said axis due to said axial focusing field, said interaction cells being axially determined by conductive end walls transverse to said axis and having ofi-axis, kidney-shaped coupling apertures relieved therethrough for intercoupiing adjacent ones of said interaction cells, said coupling apertures associated with each of said end walls having a pre determined azimuth angle of disposition about said axis with respect to an axial reference plane, said azimuth angles associated with a sequential series ofsaid conductive vanes being distributed as follows: said azimuth angle of a reference vane being zero; said azimuth angle of the next sequential .vane being greater than zero and less than +1r radians; said azimuth angle for the second vane from said reference vane being again zero; said azimuth angle for the third vane from said reference vane being between zero and :r radians; said azimuth angle of the fourth vane from said reference vane being again zero whereby said coupling apertures are in a zigzag azimuthal orientation about said axis along the length of said tube for precluding synchronous interaction between fields of said electron stream and said azimuthally varying mode of excitation.

References Cited in the file of this patent UNITED STATES PATENTS 2,636,948 Pierce Apr. 28, 1953 2,637,001 Pierce Apr. 28, 1953 2,842,705 Chodorow July 8, 1958 2,847,607 Pierce Aug. 12, 1958 

